Making water move How it is mixed & transported

Similar presentations

Presentation on theme: "Making water move How it is mixed & transported"— Presentation transcript:

1 Making water moveHow it is mixed & transportedVertical transport & mixing achieved by differences in density of different seawater massesWhat effects the density of seawater?TemperatureSalinityDensity affected more bytemperature than salinity,because the former is morevariable in the ocean

2 Vertical mixing in the oceanDensity profile with depth at tropical and polar latitudesRemember the thermal profiles with depth at tropic, temperate and polar latitudesPolar regions essentially isothermal to the bottom, with coldest temperatures near the surface in winterIn temperate & tropical regions a warmer surface layer lies above a deep, cold layerWarm water less dense than coldIn a density-stratified water column, warm surface water floats on colder water belowTemperate & tropical regions stable, polar regions-density unstable

3 Dense surface waters at the poles sinkAt the poles in winter, cold surface water is more dense than underlying warmer waterMoreover, formation of sea ice at high latitudes concentrates salinity further contributing to increased densityThe greater density of cold, saline polar waters causes them to sink and form the water at the bottom of all the world’s oceans at all latitudes by moving towards the equatorIt takes hundreds of years for a water mass sinking at the poles to complete its journey

4 Deep Ocean CirculationAll circulation below 1000 m is accomplished principally by density differences of polar water masses and their transport both down in the water column and to lower latitudesThe density of a particular water mass determines what layer, or depth beneath the surface it resides at

5 Surface ocean circulationWhat moves water horizontally across the ocean??

6 Surface ocean circulationWhat moves water horizontally across the ocean?WindCoriolis effect (due to the earth being a sphere that rotates from west to east)The Coriolis force is that of the earth’s rotation on the movement of particles

7 Coriolis force • Particles moving toward the equatormove from low to high eastwardvelocity; this lag deflects them tothe westA particle at the equator travels east at 1613 km/h to complete a full rotation in one dayParticles nearer to the poles travel eastward much more slowly because the circumference is lessSo, particles traveling from the equator towards the poles move from areas of high to low eastward velocityThey will have a relative deflection further east

8 Coriolis forceWhether moving towards or away from, the equator, the effect is the same:In the northern hemisphere, parcels of water are defelcted to the RIGHTIn the southern hemisphere, parcels of water are deflected to the LEFT

9 Combining the effects of planetary winds and Coriolis in the movement of oceanic surface watersPlanetary wind system driven by differential heating of earth’s surfaceSolar insolation is greatest near the equator and least near the polesA given amount of sunlight spread over a larger area if it strikes at an angle: therefore, lower density of energySolar radiation must pass through more atmosphere at poles than at equatorBecause of earth’s tilt, this pattern varies seasonally

10 Building a model of oceanic circulationPlanetary wind system driven by differential heating of earth’s surface•Solar insolation is greatest near the equator and least near the poles-A given amount of sunlight spread over a larger area if it strikes at an angle: therefore, lower density of energy-Solar radiation must pass through more atmosphere at poles than at equator•Because of earth’s tilt, this pattern varies seasonally

11 Building a model of oceanic circulation•Warm air rises, cold air sinks•Air at equator heated, so rises3060NS

12 Building a model of oceanic circulation3060NS•As surface air rises, it is replaced bysurface air flowing from north & south•Risen equatorial air expands as pressureis less high in the atmosphere•Expanded air cools and starts to descendbut is pushed north and south by airrising behind it.•As air high in the atmosphere is deflectednorth or south, coriolis acts on it pushing iteastward; consequently it sinks to the surfaceat ~30° latitude•This air warms as it descends (takes up moisturewhich is why the world’s deserts predominate atthese latitudes

13 Building a model of oceanic circulation3060NSBuilding a model of oceanic circulation•The first cell, or tropical cell,drives the other 2 cells whichare more or less passive•Blue arrows represent the windsblowing across the earth’s surface(on globe showing effect of coriolisforce on wind with red arrows)•As wind blows over the surfacewater, the water (because of Coriolisforce) does not travel in the samedirection as the wind•Water is deflected at an angle of 45°relative to the wind

14 Ekman SpiralFriction of the surface water layer acting on layers beneath it, causes underlying layers to be dragged in motion.They too are deflected by the coriolis forceAs the influence of wind is greatest at the air-sea interface, the effect decays exponentially with depth and eventually endswind45°Surface current

15 Ekman TransportThe sum of these vectors over all depths results in a direction of net transport of water of 90° to that of the wind90° to the right of the wind in the northern hemisphere90° to the left of the wind in the southern hemispherewindSurface current45°Ekman transport

17 Building a model of oceanic circulation•Subsurface flow of water from highto low areas of oceanic pressure•Water masses deflected, as always,by Coriolis force•Results in mid-latitude, highpressure oceanic gyres in bothnorthern & southern hemispheresdue to the presence of landmasses.NLHLHLS

18 Comparisons between ocean basinsHHHH• Both oceans have equatorial currents that flow west• An equatorial countercurrent that runs east along the equator• Oceanic high pressure systems at 30° N&S that generate gyres in the ocean• Polar current systems

19 Differences between N & S hemispheresMore land in northern hemisphere results in smaller polar current systems in both Pacific & Atlantic compared to southern hemisphereIn southern hemisphere, the west wind drift is circumpolar because virtually no land in its path at 60°S latitude; not only surface moving east, but sub-surface geostrophic flow as well.Western intensification- weaker in south Pacific because of a leaky boundary associated with IndonesiaEastside boundaries - at 60°S latitude, swiftly, moving open ocean to east because there is no land, except area between Palmer Peninsula of Antarctica and South America;a string of islands that that act as a weir blocking free flow of waterThis pile up of water is deflected north and becomes the exceptionally strong Humboldt (or Peru-Chile) current